Fiber optic pressure sensor systems

ABSTRACT

The invention is related to numerous improvements for fiber optic measuring systems, and principally those utilizing a deformable diaphragm for sensing pressure. One feature of the invention is particularly oriented to automotive engine combustion chamber pressure measurement. For that application, a sensing tip shielding technique is described using a sintered metal porous plug, which reduces temperature fluctuations experienced at the sensing tip and shields the sensing tip from corrosive materials. A technique for compensation of a fiber optic pressure sensor is also described. In that method, a referencing wavelength is partly reflected and partly transmitted past a filter coating at the sensing end of the optical filament. This technique of the sensor provides improved calibration.

This is a division of U.S. patent application Ser. No. 748,082, filedAug. 21, 1991 entitled: Fiber Optic Pressure Sensor Systems, now U.S.Pat. No. 5,275,053.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to improvements in fiber optic sensor systems,and in particular, to the provision of temperature measurement andcompensation mechanisms, calibration systems, and additionalimprovements both in operating methodologies and design features.

Optical fiber sensing systems have found applications in variousenvironments. For example, the measurement of intravascular bloodpressure of human patients has been accomplished using equipmentmanufactured by the present assignee, FiberOptic Sensor Technologies,Inc. (FST) in which a diaphragm at the fiber sensing tip deforms inresponse to pressure differentials, and thus modulates through areflection, a light signal sent through the fiber. Changes in thedistance between the deformed diaphragm and the optical fiber end, andthe diaphragm shape, modulate the amplitude of light that is reflectedback into the optical fiber. Accordingly, the intensity of the returnedlight signal is related to pressure acting on the sensing tip.

Applicants have made numerous advancements in the technology of fiberoptic sensing systems which are principally oriented toward pressuremeasurement. The present assignee, FST also owns U.S. Pat. Nos.4,711,246; 4,787,396 and 4,924,870, all related to various improvementsin fiber optic sensors, and which are hereby incorporated by reference.While the systems in accordance with these prior patents provideexcellent performance for the intended applications, applicants areseeking to enhance the application environments which fiber opticpressure sensors may be used in.

One particularly demanding application for a pressure sensor is that ofsensing within an internal combustion engine combustion chamber. Thereare presently numerous sensor approaches toward conducting such pressuremeasurement to provide real time measurement of combustion chamberpressure, which information can be used for controlling engine operatingparameters such as spark timing, air/fuel ratio, exhaust gasrecirculation (EGR), etc. to optimize engine performance. However, suchan application is an extremely demanding one for a sensor. Extremetemperatures and temperature ranges would be encountered during use,with high accuracy and low cost required for such a product. Moreover,the combustion chamber environment exposes the sensor to intenseelectromagnetic fields, mechanical shock, and a corrosive atmosphere.

In assignees previously issued U.S. Pat. No. 4,924,870, a technique forcompensating a fiber optic measuring system was described. That systemis particularly adapted for compensating the pressure reading output ofthe device with respect to differences in outputs of the light sourcesused to inject light pulses into the fiber, fiber-to-fiber variations,and the bending effect (i.e. loss of signal resulting from curvature ofthe fiber). The previously described calibration scheme operates byusing a reflective coating at the sensing tip end of the fiber which isreflective to a calibration light signal which is returned along thefiber, whereas the pressure measuring light signal is transmittedthrough the coating and is modulated by the pressure responsivediaphragm. That calibration scheme is capable of calibrating thepressure responsive light signals for all of the principal variablesaffecting response along the length of the optical fiber. That system isnot, however, capable of compensating for parameters aside from pressurewhich affect the pressure measuring light signal beyond the filamentend. Accordingly, although that "dual wavelength" scheme operatesextremely well in environments where temperature ranges are low, andwhere single uses are contemplated, it has limitations in environmentswhere extreme temperature variations and time dependent changes can beanticipated.

Temperature is the primary source of inaccuracies of high temperaturefiber optic pressure sensors operated on the principal of a flexingdiaphragm. Temperature fluctuations and extremes affect each of the fourprimary areas of these sensors including, the: sensing diaphragm;sensing tip; fiber optic link; and the associated opto-electronics andelectronics. In particular, high temperature extremes, such asencountered in automotive engine combustion chambers at the sensingdiaphragm, and somewhat lower temperature fluctuations at the sensingtip, are the two dominating sources of errors. Large temperatureextremes change mechanical properties of the diaphragm, mostly Young'smodulus, resulting in temperature dependent deflection. Temperatureinduced expansion of the elements of the sensing tip change the relativeposition of the fiber end and the diaphragm, and causes transmissionchanges through the fiber.

One feature of this invention is a technique for temperaturecompensation in diaphragm-based fiber optic pressure sensors. Thetechnique is designed to eliminate, or significantly reduce, undesiredtemperature effects on the deflecting diaphragm, sensor tip, opticallink, and electronic interface. An underlying principal of thecompensation technique described in this specification is thesimultaneous measurement of diaphragm deflection and sensing tiptemperature, and real time software correction for the temperatureeffect.

In accordance with this invention, several methods for temperaturemeasurement are described for enabling temperature compensation. In afirst approach, a temperature sensing light signal is chosen to have awavelength at near the cut-off wavelength (i.e. boundary between highreflectivity and high transmissivity) of a filter coating on the opticalfiber end at the sensing tip. Since the cut-off wavelength of amultilayer dielectic film filter changes in accordance with temperature,the intensity of a reflected back signal for a temperature compensatinglight signal can be used as a measure of temperature. The wavelength canalso overlap a region of a sharp peak in transmissivity which is oftenfound in such filter coatings. Since the wavelength of such a peak willshift in response to temperature it also provides a convenientopportunity for temperature measurement.

In another technique for temperature compensation according to thisinvention, two light signals are injected into the optical fiber havingdifferent launching conditions. It has been found that certaintransmission modes are affected by temperature in a differential mannerfor applicants' sensor systems. For example, a launching condition inwhich the injected light intensity is concentrated mostly along theouter surface of the fiber is attenuated in response to highertemperatures more so than a mode which is concentrated at the center ofthe fiber, which is only weakly coupled to the fiber outer surface.

Another facet of this invention is a technique for reducing thedistortion effect of temperature variation and other factors beyond theoptical filament end which does not require actually measuringtemperature. This technique is used with dual wavelength systems asdescribed in assignees U. S. Pat. No. 4,924,870 and involves allowing acalibration light signal to be partially transmitted and partiallyreflected by the filament end filter coating. This approach can be shownto reduce the span of errors which are not directly compensated forthrough the dual wavelength system.

In addition to the temperature effects mentioned previously,applications in which the same sensor is expected to providemeasurements over a period of time gives rise to concern over timedependent changes in the sensing tip. For example, the reflectivity ofthe deformable diaphragm caused by oxidation or other effects candramatically alter the intensity of the returned light signal, andhence, affect measurement. In accordance with this invention, acalibration scheme based on zero-setting the system when it is exposedto a known pressure is provided which is specifically oriented to anautomotive application.

This specification further describes a temperature shielding feature forfiber optic pressure sensor tips exposed to high temperatures, such asthose found in internal combustion engines. By reducing the range oftemperatures to which the sensor tip is exposed, some of the temperaturedependent effects can be reduced.

In applications where multiple pressure sensors are used, this inventionfurther contemplates techniques for multiplexing a number of identicalsensors performing simultaneous pressure measurement.

Additional benefits and advantages of the present invention will becomeapparent to those skilled in the art to which this invention relatesfrom the subsequent description of the preferred embodiments and theappended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a pressure sensing system accordance with afirst embodiment of this invention.

FIG. 2a is a wavelength versus transmission spectrum showing threedifferent wavelength sources and the transmission characteristic of aoptical filament filter coating used to provide temperature compensationin accordance with an embodiment of this invention.

FIG. 2b is a wavelength versus transmission spectrum showing threedifferent wavelength sources like that shown in FIG. 2a but showing apeaked transmission response of the filter coating overlapping onesource for temperature sensitivity.

FIG. 3 is a wavelength versus transmission spectrum showing a method fortemperature compensation based on differential temperature modulation oflight signals based on their launching conditions.

FIG. 4 is a pictorial view showing a light source providing a launchingcondition for an inputted light which produces maximum intensity nearthe fiber outer surfaces.

FIG. 5 is a pictorial view showing a light source providing a launchingcondition for an inputted light which produces maximum intensity nearthe fiber center.

FIG. 6 is a cross sectional view of a sensing tip of this inventiondefining a closed gas volume acting on one side of the sensingdiaphragm.

FIG. 7 is a pictorial view of a fiber optic pressure sensing tip withcable and optical connector in accordance with this invention whichprovides a means for shielding the deformable pressure sensing elementof the sensing tip.

FIG. 8 is a wavelength versus transmission distribution for a systemaccording to an embodiment of this invention for reducing calibrationerror by allowing a portion of the reference light signal to leak pastthe fiber end coating in which the transmission curve has an elevatedminimum transmission characteristic.

FIG. 9 is a wavelength versus transmission distribution for a systemsimilar to that of FIG. 7 except that the filter cut-off wavelengthpartly overlaps the reference light source spectrum.

FIG. 10 is a curve showing a reduction in error attributable tooperation in accordance with FIGS. 7 or 8.

FIG. 11 is a schematic drawing of a multiplexing system in which acommon photodetector is used to monitor the output from plural sensors.

FIG. 12 shows an optical shutter system used in conjunction with thesystem of FIG. 11 using optoelectric shutters.

FIG. 13 shows an optical modulation system used in conjunction with thesystem of FIG. 11 using piezoelectric microbend attenuators.

FIG. 14 is a schematic drawing of multiplexing system in which a commonset of light sources is used with designated photodetectors for eachsensor.

DETAILED DESCRIPTION OF THE INVENTION

This specification is directed to a number of improvements for fiberoptic sensing systems. For the sake of clarity, the various categoriesof aspects of this invention are discussed in separate sectionsidentified by descriptive headings. Some of the various embodiments,however, share common elements which are identified by like referencenumbers.

Temperature Measurement and Compensation

A generalized fiber optic pressure sensing system is shown in FIG. 1 andis generally designated there by reference number 10. System 10 includesfiber optic cable 12 which has a sensing tip 14 at its terminal end.Optical filament 11 passes through ferrule 13 to which it is bonded orinterference fit. Cylinder 15 surrounds ferrule 13. Deflectablediaphragm cap 16 has a center membrane portion which deforms in responseto pressure differentials across it, and thus changes the amplitude oflight launched into optical filament 11 at the opposite end of thefilament which is returned back into the filament. A vent passage 19 isprovided to maintain the fiber side of the diaphragm cap 16 at a desiredpressure. A partially reflective dielectric filter 17, comprised ofnumerous layers of dielectric material, for example titanium dioxide andsilicon dioxide, is deposited on the end of filament 11 which provides adual wavelength referencing feature such as described in applicant'spreviously issued U.S. Pat. No. 4,924,870.

A temperature compensating system using optical means of temperaturemeasurement according to an embodiment of this invention is shown inFIG. 1. The system 10 consists of an opto-electronic interface 18containing three LEDs 20, 22 and 24, each emitting light in distinctwavelength bands; photodetectors 26 and 28; and a coupler (not shown)for launching light from the LEDs into optical filament 11, anddirecting the returned signals to be incident on the photodetectors.Some of the light from the LED's is directed to fall on photodetector 26which provides a source intensity reference. Three output signals fromthe opto-electronic interface 18 are converted into a digital form by Ato D converter 28 and transmitted to digital signal processing (DSP)module 34. These three signals are related to the intensity of thereturned signals of the three LEDs. The outputs of module 30 arepressure and temperature readings.

The DSP module 34 and associated host microprocessor control theoperation of LEDs 20, 22 and 24, and perform real time computations andconversions. A computation algorithm is used to deconvolve temperatureeffects from the pressure output and is derived from the fundamentalrelationships between diaphragm deflection, pressure, and temperature.Experimentally derived correction factors may be also used as inputs tothe compensation algorithm.

The deflection of the diaphragm cap 16 is measured using the dualwavelength technique described in U.S. Pat. No. 4,924,870 in which thelight signal from LED 20 is reflected by filter layer 17, whereas thelight pulse from LED 24 passes through the filter to be modulated bydiaphragm cap 16. The use of the referencing wavelength from LED 20 inthis approach permits compensation for environmental effects on thesystem such as vibration, connector instability, bending effects, andsome of the sensing tip sensitivities.

Fiber optic temperature measurement of the sensor tip 14 is performed ina novel way according to this invention and does not require anymodifications to the sensor designed for dual wavelength operation butinstead only requires modifications to interface 18. This is animportant consideration for price sensitive applications, such as inautomobiles, where the price of replaceable elements has to be very low.In essence, temperature is measured by monitoring the intensity of thethird wavelength produced by LED 22 returned into the fiber at sensingtip 14, and time division multiplexed the signal from photodetector 28with the other two wavelengths. This temperature sensitive wavelength isselected such that the corresponding signal is most sensitive totemperature changes at the sensor tip 14. One location for thewavelength is between the other two wavelength bands produced by LEDs 20and 24, and overlapping with the cut-off wavelength of filter 17.

In FIG. 2a, the output spectra of LEDs 20, 22 and 24 are shown as curves36, 38 and 40, and can be designated as the reference, temperature andpressure sensitive wavelengths, respectively. Under varying temperature,the cut-off slope of filter 17, designated by curve 42, moves around itsnominal position at a temperature T₂. As temperature increases to T₁ thecut-off slope moves towards shorter wavelengths, increasing theintensity of the transmitted temperature sensing wavelength(lambda_(t)). If the coating parameters are correctly selected, thetransmission levels of the reference wavelength light (lambda_(r)) andthe pressure sensitive wavelength light (lambda_(p)) change very little.

The cut-off slope of the transmission curve is not the only temperaturesensitive range of the curve. A region of the curve exhibiting theside-lobe behavior, as shown in FIG. 2b designated by reference number43, exhibits possibly even higher temperature sensitivity because it istypically narrow than the line-width of LED 20 and it shifts by the sameamount that the cut-off slope does. By overlapping the temperaturemeasuring wavelength 36 on the side-lobe 43, an alternative temperaturemeasuring technique is possible, as shown schematically in FIG. 2b.

The temperature dependent changes in lambda_(t) are obtained by taking aratio of the signals at lambda_(p) and lambda_(t) where:

    v(t)=V.sub.t (p,t)/V.sub.p (p)

and

    V.sub.t (p,t)=V.sub.t (t)*V.sub.t (p)

V_(p) (p,t) is the intensity of lambda_(p), and V_(t) (p,t) oflambda_(t). Note that V_(t) is modulated by both pressure andtemperature, however the ratio v_(t) depends only on temperature. Inoperation, the LEDs 20, 22 and 24 would be fired sequentially with theoutput of photodetector 28 gated to ensure output synchronization.

Several issues have to be addressed to ensure the practicality of theabove described techniques. For typical LEDs, such as those outputtinglight in the following wavelength bands with maximum wavelengths at; 730nm, 840 nm, and 950 nm, their line widths may have to be restricted byband-pass filters (not shown). The purpose of such filters is to reducethe spectral overlap of the LEDs and consequently improve the accuracyof the temperature measurement.

Temperature sensing is also possible within a flat portion of thetransmission curve as shown in FIG. 3. A necessary condition is that thetemperature sensing wavelength is differently affected by temperaturethan the referencing wavelength. A novel technique, and an alternativeto the technique described above, is to differentiate the temperatureeffect between the referencing and temperature sensing wavelengthsthrough the differentiating of the launching conditions between the twowavelengths. This can be accomplished in a simple manner inoptoelectronic interface 18 by restricting the numerical aperture of thepressure and reference sensing LEDs (underfilling the launch) oroverfilling the modes for the measuring wavelengths. For short fibers,such as of interest here, the launching condition at interface 18 isvery well preserved all the way to sensing tip 14 and back to theinterface such that the two wavelengths have different modaldistributions. This inventor has found that the light signals aredifferentially affected by temperature. For the temperature changes ofthe order of hundreds of degrees Centigrade, relatively smalldifferences in launching condition result in significantly differenttemperature sensitivities. As an alternative embodiment, two LEDs havingidentical wavelength outputs could be used if they are launchedsignificantly differently into the fiber.

A different launch condition may result, in principle, in somewhatdifferent sensitivity to filament bending, and fiber or connectorinstabilities. However, temperature effects at sensing tip 14 are solarge that the launching mismatch needed for temperature measurementdoes not significantly affect sensitivities to other environmentaleffects.

FIGS. 4 and 5 provide simplified illustrations of techniques forproviding different launching conditions for two signals which aredifferentially affected by temperature. In FIG. 4, LED 44 is shownprojecting light through aperture 46 and into fiber optic filament 11.Aperture 46 has the property that it blocks light from entering at nearthe center longitudinal axis of filament 11 and, therefore, produces amaximum intensity distribution which is near the radially outer surfaceof the filament. As mentioned previously, this launching condition iswell preserved along filament 11 and, therefore, an intensitydistribution of light through any cross-section of the filament willreveal peak intensities near the outer surface of the filament. FIG. 5is similar to FIG. 4, except showing aperture 48 which produces amaximum intensity along the central longitudinal axis of filament 11. Inthat launching condition, a maximum light intensity distribution will befound around the center core area of the filament. While variousexplanations for a differential temperature effect between the launchingcondition providing by FIG. 4 versus FIG. 5 can be provided, one theoryis the mechanical stresses generated along the outer surfaces offilament 11 cause distortion in the outer surface of the filament,especially in the area of contact with ferrule 13, leading to greaterleakage of the off-axis launched light of FIG. 4, as compared with theon-axis light of FIG. 5, where such surface phenomena have a relativelysmall effect.

The effectiveness of the temperature compensation described in thisinvention depends on the ability of accurately relating the signalsv_(p) and v_(t). In principal, these signals are coupled through threeeffects: the fundamental thermo-mechanical properties of the diaphragmcap 16 material, the temperature expansion of the elements of sensingtip 14, and the pressure and temperature dependent transmission changesof the optical signals. A temperature compensated pressure reading isobtained by what is equivalent to a real time inversion of thefunctional dependence between the changes in optical signal anddiaphragm deflection, which in turn depends on both temperature andpressure as follows:

    v.sub.out (p)=F.sup.-1 {v.sub.t [v.sub.d (p,t),t]}

This inversion is realized through the use of a two-dimensional look-uptable or curve fitting techniques. In practice, it is very difficult topredict exactly the functional behavior of v_(t). Instead, v_(t)dependence on pressure and temperature is stored in the memory of thesystem during a calibration process when sensing tip 14 is subjected tofull operational temperature and pressure ranges.

A particular look-up or curve fitting technique is implemented based ontwo primary factors: the degree of the accuracy of the theoreticalmodel, and the accuracy of the deflection and temperature readings. Inan ideal situation, the correction factors are identical for all sensorsand identical correction parameters are stored in the memory of thesystem based on an unique inversion relationship. In practice, however,some kind of individual sensor calibration needs to be institutedbecause of the manufacturing variability. A two (or more) pointcalibration technique could be implemented.

Periodic Pressure Calibration

Time dependent changes in the optical system of sensor 10 can beexpected to occur resulting for example, from reflectivity changes ofdiaphragm cap 16, and changes in the transmission of the opticalfilament 11. To correct for these effects, a zero-offset technique canbe implemented. In the application of measuring pressure within anautomotive engine combustion chamber, "re-zeroing" of the sensorpressure output can be provided during the engine shut-off periods whenthe combustion chamber pressure is equal to the atmospheric pressure.Because combustion pressure does not instantaneously reach atmosphericpressure after the engine shuts down, alternative verification of thepressure equilibrium is needed. One way of doing so is to wait a certainamount of time after engine shut down. The necessary delay can be easilydetermined and stored electronically. In an automobile, this re-zeroingcan also be performed during the regular engine status check precedingengine ignition.

Sealed Cavity Design

Due to prolonged use and high temperature in some applications such ascombustion chamber pressure monitoring, the reflecting diaphragm 16, theoptical coating 17, and the bonding material used may deteriorate overtime. Periodic calibration described above will be effective if theresulting changes in optical signals are smaller than required by thedynamic range of the system. To minimize these changes, a technique ofsealing of the area between the fiber 11 end face and diaphragm 16 isdescribed here. As shown in FIG. 6, the area behind diaphragm 16 is notvented to atmospheric pressure. If a gas is used to fill this space,temperature dependent back pressure changes will result. However, thistemperature effect is well behaved and will be compensated for by thetemperature compensation technique described in this specification. Aninert gas, such as for example argon, would be used eliminating anyresidues of oxygen and water vapor that are the main causes of potentialoptical changes inside the sensor. The sealed gas will exert pressure ondiagram cap 16 as dictated by the Ideal Gas Law. The changes in thispressure would be a component of a lumped correction parameter.

As a modification to the above approach of providing a sealed cavitybehind diaphragm 16, a vent passage 30 as shown in FIG. 1 could be usedto enlarge the sealed volume. In such a configuration, vent passage 30would be plugged along fiber optic cable 12 along its length or atinterface 18. The advantage of such a configuration is that for someapplications, such as combustion chamber pressure measurement, theaverage temperature of the sealed gas would be closer to ambienttemperature, thus reducing the pressure change acting on diagram 16dictated by the Ideal Gas Law, and further reducing the rate oftemperature change of the trapped gas.

Sensing Tip Shielding

Another critical issue for long term accuracy and reliability of a fiberoptic sensor system is the mechanical stability and durability of thematerial of diaphragm cap 16. In an automotive application, a diaphragmdirectly exposed to the combustion chamber may be subjected totemperature between -50 degrees C and 550 degrees C. This represents aformidable environment for the most temperature resilient metals.

As a part of this invention, a technique for temperature shielding ofthe diaphragm is described with specific reference to FIG. 7. In FIG. 7fiber optic cable 12 is shown specifically adapted for incorporationinto an automotive internal combustion engine for measuring combustionchamber pressure. Fiber optic cable 12 is shown housed within mountingplug 52, which is threaded to be received in an associated port exposedto the combustion chamber. As an alternative design, cable 12 could beincorporated into an ignition spark plug. Mounting plug 52 has aninternal cavity for supporting fiber optic cable 12. The opposite end ofthe cable 12 has an optical connector 54. A shield 56 is provided,positioned between sensing tip 14 and the combustion chamber. Shield 56is made from sintered material such as that frequently employed inpressure lines. Such a material is made of pressed metal bids (can bestainless steel or brass) which form a porous structure of very highheat mass capable of rapid temperature increases. Shield 56 is in theform of a disk, a few millimeter thick, thermally coupled to mountingplug 52 and reduces temperature extremes by hundreds of degrees.Furthermore, this material does not reduce the frequency response of thesensor. By selecting the size and material of the bids, disk 56 mayadditionally attenuate high frequency jitter on the pressure waveformsas may result from the presence of the cavity above the sensordiaphragm. An additional benefit of placing shield 56 in front ofsensing tip 14 is protection of diaphragm cap 16 against formation ofdeposits on its surface. Such deposits may alter the diaphragm's dynamiccharacteristics over prolonged operation and result in sensorinaccuracies.

Sensing Tip Compensation

The dual wavelength technique according to U.S. Pat. No. 4,924,870 anddiscussed previously for fiber optic pressure sensors permitscompensation for environmental effects on the sensor including fiberbending, connector mechanical and thermal instabilities, and temperatureinduced changes in fiber transmission. However, temperature effects, andany other effects, occurring between the sensor diaphragm and the coatedsurface of optic filament 11 affect only the measuring wavelength andmay result in sensor tip sensitivity and drift. This error may berelatively small if the temperature effect on transmission beyond filtercoating 17 is small even if the sensitivity for the signals reflected bythe coating is large (because this sensitivity is the same for the twowavelengths provided that their launching conditions are identical).Yet, if the temperature sensitivity beyond coating 17 is significant, asfrequently encountered due to manufacturing variabilities, the error maybe significant.

This aspect of the invention relates to an improvement to the dualwavelength technique that results in significant reduction in thedifferential effect at the tip and overall reduction in sensor error.This benefit is achieved without a direct measurement of sensing tip 14temperature, as is conducted in accordance with the prior embodiments.

The basic principal of this embodiment of the invention is allowing thereference wavelength light signal to partially leak through coating 17and benefit from the resulting larger reduction in the differentialeffect at sensing tip 14 compared to the reduction in the full scalepressure change (span). Referring to FIG. 8, the transmission curve ofcoating 17 is modified compared to the original profile shown in FIGS. 2and 3 such that the reference signal is partially transmitted. Twoapproaches are depicted by the figures for allowing the reference lightsignal to be partially transmitted. In FIG. 8, the modified transmissioncurve 58 of filter 17 is shown in which the transmissance of the filterdoes not reach zero within the range of wavelengths of interest. Asshown in FIG. 8, a large proportion of the total energy of the referencewavelength is transmitted past filter coating 17. In FIG. 9, partialleakage of the reference wavelength light signal is achieved bypartially overlapping the cut-off range of filter 17 with the referencewavelength signal, using a filter with the characteristics of curve 60.Care has to be exercised in selecting an optical source such thatlaunching condition is essentially the same for both the reference andpressure sensing wavelengths. Low bend sensitivity can be used as a testof good launch overlap.

The benefit of leaking a portion of the reference wavelength lightsignal stems from the dependence of the ratio of tip sensitivity andspan. The intensity of the two signals can be expressed as:

    V.sub.i =I.sub.i F.sub.i (R.sub.i +T.sub.i.sup.2 D)

Where I_(i) is the intensity of the signals, F_(i) is the transmissionfactor representing the lumped transmission coefficient of filament 11up to the fiber end, R_(i) and T_(i) are the reflection and transmissioncoefficients, respectively, of the coating, and D is the pressuredependent reflection coefficient of the diaphragm. The voltage output ofthe sensor is proportional to the ratio of the signals of the previousequation.

    V.sub.o =V.sub.m /V.sub.r =(R.sub.m =T.sub.m.sup.2 D)/(R.sub.r =T.sub.r.sup.2 D)

For small changes, the difference between the temperature inducedeffects on the reference and pressure sensing wavelengths can beexpressed as:

    W=dI.sub.m /I.sub.m -dI.sub.r /I.sub.r =(r+2T.sub.m t D)/(R.sub.m +T.sub.m.sup.2 D)-(r+2T.sub.r t D)/(R.sub.r +T.sub.r.sup.2 D)

where r=dR^(i) /dt, t=dI_(i) /dt

The span of the sensor can be expressed, in a small changeapproximation, as

    s=T.sub.m.sup.2 /(R.sub.m =T.sub.m.sup.2 D)-T.sub.m.sup.2 /(R.sub.m +T.sub.m.sup.2 D)

The expression for total error as a fraction of span is expressed as:

    Error=W/S

The dependence of the error term on the transmission coefficient T_(r)is schematically shown in FIG. 10 together with the dependencies of thespan and tip sensitivity. Relative units are used in showing theserelationships. FIG. 10 illustrates that increases in transmissioncoefficient results in lower error.

Sensor Multiplexing

Finally, this disclosure describes techniques of multiplexing a numberof identical sensors 14a through d that can perform simultaneouspressure measurements. Such measurements are needed for automotiveengine controls based on combustion pressure measured in individualengine cylinders. Two schemes are described here: a first that utilizesa single measuring photo detector as shown in FIG. 11, and a secondbased on multiple of measuring detectors, shown in FIG. 14.

The multiplexing scheme shown in FIG. 11 is a combination of threetechniques: wavelength, time, and frequency multiplexing. Wavelength andtime division multiplexing are used as previously described, forcompensating for temperature and environmental effects acting on anindividual sensor. Frequency multiplexing is employed to multiplexdifferent sensors shown as 14a through d without increasing thecomplexity and cost of the opto-electronic interface 18. This modulationis realized by using electronically driven shutters integrated within anin-line connector 64 which are operated by controller 74.

Various modulation techniques could be used. One approach, as depictedin FIG. 12, is based on four piezoelectrically driven shutters 66athrough d introduced between the ends of interconnecting fibers 11athrough d. This technique requires the use of an expanded beam connectorbetween the fibers 11a through d and mixing rod 70 to minimizetransmission losses resulting from separation of the fibers.Alternatively, as shown in FIG. 13, piezoelectric squeezer shutters 68athrough d apply pressure alongside the fiber. If the squeezer 68 has atooth-like shape, intensity modulation can be induced through themicrobending effect. It is further contemplated that liquid crystalshutters could be employed. This approach eliminates the use ofmechanical devices and ensures long time reliability.

The shutters 66 or 68 can operate in two alternative modes. One moderequires that all four shutters continuously close and open, each atdifferent frequency. The signals of different frequencies are separatedthen using phase sensitive detection. Such a detection technique offersadditional benefits of improved signal-to-noise ratio.

An alternative approach requires opening and closing the shutters 66 or68 sequentially. Each of the sensors 14a through d is interrogated at adifferent time in essentially the same manner as in the case of anindividual sensor.

Each of the four channels comprises fibers 11a through d carry the threetime division multiplexed signals corresponding to the three wavelengths36, 38 and 40. These signals are processed by the DSP module 34 toextract the pressure and temperature information for each sensor 14athrough d. Four pressure outputs from DSP module 34 are transmitted thenin an appropriate format to an Electronic Control Module (ECM) of anautomobile.

The second approach, being the subject of this disclosure isschematically depicted in FIG. 14. A time division multiplexed multipledetector design is used. Each of the three LEDs 20, 22 and 24 isconnected, via a fiber lens interface 72, to each of the four sensingfibers 11a through d. The return signal from each of the fibers 11athrough d is detected by individual measuring detectors 28a through d,provided for each sensor 14a through d. The signals of the fourdetectors 28a through d are processed independently, synchronized to theemitting LEDs 20, 22 and 24 the same way it is done for a single channelsensor described herein.

While the above description constitutes the preferred embodiments of thepresent invention, it will be appreciated that the invention issusceptible of modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

We claim:
 1. A fiber optic pressure sensor system comprising:an opticalfiber having a sensing tip at a first end with a pressure sensitiveelement, said pressure sensitive element modulating a pressure sensinglight signal injected into the opposite second end of said opticalfiber, optical means for generating said pressure sensing light signaland a reference light signal, said light signals having differentwavelengths, filter means at said fiber first end for allowingtransmission of said pressure sensing light signal and reflecting aportion of the energy of said reference light signal allowing atransmitted portion of said reference light signal to be modulated bysaid pressure sensitive element, and signal processing means forgenerating an output related to pressure at said sensing tip and wherethe amplitude of both returned reference and pressure sensing lightsignals are used to generate an output related to pressure at saidsensing tip.
 2. A fiber optic pressure sensor system according to claim1 wherein said sensing tip includes a diaphragm positioned beyond saidfirst end of said fiber which is deformable in response to pressurewhereby the energy of said light signals reflected by said diaphragm andreturned into said fiber is modulated by said pressure.
 3. A fiber opticpressure sensor system according to claim 1 wherein said filter means ispartially transmissive and partially reflective with respect to saidreference light signal.
 4. A fiber optic pressure sensor systemaccording to claim 1 wherein said filter means defines a cut-offwavelength band defining the wavelength wherein said filter transitionsbetween light transmission and reflectivity and wherein said referencelight signal is defined by a band of wavelengths which overlap saidcut-off wavelength band wherein a portion of said reference light signalis reflected by said filter and a portion is transmitted through saidfilter.
 5. A fiber optic pressure sensor, comprising:an optical fiberhaving a sensing tip at a first end with a pressure sensitive element,said pressure sensitive element modulating a pressure sensing lightsignal injected into the opposite second end of said optical fiber andreturning said light signal into said fiber, and a shield for saidsensing tip positioned between said pressure sensitive element and theenvironment in which said pressure measurements are desired, said shieldformed of a porous metallic structure.
 6. A fiber optic pressure sensoraccording to claim 5 wherein said pressure sensitive element comprises adiaphragm which is deformable in response to pressure thereby modulatingthe intensity of said pressure sensing light signal inputted into saidfiber which is returned by reflection of said diaphragm.
 7. A fiberoptic pressure sensor according to claim 5 wherein said sensing tip ispositioned to communicate with an internal combustion engine combustionchamber.
 8. A fiber optic pressure sensor according to claim 7 furthercomprising mounting means for securing said optical sensing tip and saidshield which can be threaded into a port communicating with saidcombustion chamber.
 9. A fiber optic pressure sensor according to claim5 wherein said shield is made from sintered metal bids.
 10. A fiberoptic pressure sensor according to claim 5 wherein said shield is madefrom stainless steel or brass.